1. Field of the Invention
The present invention relates to a power supply circuit used in various kinds of electronic apparatuses. More particularly, it relates to a circuit for controlling a direct-current to direct-current conversion so as to keep constant a voltage or voltages used in an electronic apparatus, and to a direct-current to direct-current conversion apparatus using the same. Note that, in the description below, xe2x80x9cdirect-current to direct-current conversionxe2x80x9d is simply abbreviated as xe2x80x9cxe2x80x9cDC/DC xe2x80x9dxe2x80x9d.
2. Description of the Related Art
A portable electronic apparatus such as a so-called hand-held type personal computer is equipped with a battery as its power source. Since the voltage of a battery is generally lowered as the battery discharges, a DC/DC apparatus is incorporated into the electronic apparatus so as to keep constant the output voltage of the battery.
On the other hand, the power source used in an electronic apparatus usually requires a plurality of power supplies, not a single power supply. To this end, a plurality of DC/DC apparatuses are provided for the plurality of power supplies. In this case, if turn-on/turn-off sequences between the respective power supplies are not fully taken into consideration, a drawback occurs in that a latch-up phenomenon is caused in semiconductor devices used in the electronic apparatus and thus some devices are burned out. Accordingly, some ideas are required for controlling power turn-on/turn-off sequences.
In one example of the prior art, the DC/DC apparatus is provided with a special logic circuit for controlling such power turn-on/turn-off sequences. However, this leads to a problem in that the scale of the entire circuit becomes large and the circuit constitution also becomes relatively complicated.
Also, where respective power supplies are simultaneously turned on with respect to a plurality of DC/DC apparatuses, it is substantially impossible to control respective rise characteristics of output voltages of the DC/DC apparatuses since the rise characteristics depend on lightness or heaviness of respective loads of the DC/DC apparatuses. A similar problem also occurs in the case where respective power supplies are simultaneously turned off.
The problems encountered in the prior art will be explained later in detail in contrast with preferred embodiments of the present invention.
An object of the present invention is to provide a DC/DC control circuit and a DC/DC apparatus using the same, by which it is possible to easily realize power turn-on/turn-off sequence controls without requiring any special logic circuit, and thus to control rise/fall characteristics of the output voltage without depending on the load.
To attain the object, the present invention is mainly characterized in that, where power turn-on/turn-off sequences are controlled between a plurality of DC/DC apparatuses (e.g., DC/DC apparatuses using a pulse width modulation (PWM) control), a plurality of reference voltages can be input to an error amplifier for voltage control provided in each DC/DC apparatus, and the rise characteristics of an output voltage obtained when the power supply to each DC/DC apparatus is turned on do not depend on the corresponding load.
Namely, the present invention is directed to devising the form of connection of a capacitor circuit for use in a soft start of the DC/DC apparatus, to thereby control the rise characteristics of the output voltage without depending on the load. Note that the explanation as to the xe2x80x9csoft startxe2x80x9d will be given later.
In the prior art, a design for the soft start control is made under the condition imagining the maximum load. Accordingly, where the load is light, the rise of the output voltage relatively becomes early, and thus it is difficult to finely control the rise of the output voltage using only the soft start control.
Contrary to this, according to the present invention, it is possible to control the power turn-on sequence without depending on the load, only by changing circuit constants of the capacitor circuit for use in a soft start of a DC/DC apparatus using a plurality of power supplies.
Also, when the power supply to each DC/DC apparatus is turned off, a difference is made between the times required until the respective output voltages reach 0 V, depending on lightness or heaviness of the respective loads and their load capacitances. In this case, if the power turn-off sequence is not properly controlled, the above latch-up phenomenon would be caused in semiconductor devices. Where such a latch-up is caused, some of the devices may be burned out.
According to the present invention, when the power supply to each DC/DC apparatus is turned off, a synchronous rectifying transistor or a load capacitance discharging transistor provided in each DC/DC apparatus is forcibly turned off to discharge charges corresponding to the load capacitance. Thus, it is possible to remove the disadvantage in that a difference is made between the times required until the respective output voltages reach 0 V.